Women with breast cancer are exposed to progestins naturally or through hormonal therapies and these have a profound effect on tumor biology including gene regulation and overall tumor growth. Progestins are breast tumor promotional in post-menopausal women taking hormone replacement therapy, though the role of progestins in established breast cancers is less clear. Progestins have been implicated in regulating growth, growth factor signaling, differentiation, and stem/progenitor properties in breast cancer cells. They act as ligands for progesterone receptors (PRs), which are expressed in over half of the ~70% luminal ER-positive cancers and signify functional ER, a favorable indicator for endocrine therapy. Progestins are thought to block estrogen/ER target genes in addition to having their own targets, and consequently are an effective therapy for some advanced breast tumors. However, there is contrary evidence that progestins stimulate the growth of some breast cancer cell lines and tumors, and there is virtually nothing known about the mechanism of its differential tumor-specific growth effects. In addition to being strong PR agonists, some synthetic progestins bind and stimulate androgen receptors (AR); AR is expressed in near 80% of breast cancers and has recently emerged as a prime therapeutic target in breast cancer. As of yet, progestin-regulated genes in human breast cancer models have only been determined using cultured breast cancer cells, not human-derived tumors. It is unknown why progestins are potently anti-proliferative in some tumors, while potentiating estrogen-induced tumor growth in others. The identification of progestin-dependent gene signatures in patient tumors may pinpoint appropriate candidates for progestin therapy in advanced tumors and/or provide a prognostic tool for predicting tumor progression. Proposed in this application is a novel method of using patient-derived luminal ER+PR+ breast tumor xenografts to define progestin-dependent gene regulation profiles that mediate tumor growth. To do this, progestin-regulated gene networks have been identified, and will be validated. These genes will be analyzed for their roles in tumor cell growth, proliferation, and apoptosis using in vitro and in vivo assays. The manipulation of progestin-regulated genes will determine their roles in progestin-mediated tumor growth in vivo. The analysis will also include elucidation of the role of AR in progestin-mediated gene regulation. Completion of this project will, for the first time, identify progestin-regulated gene signatures and the involvement of AR in individual patient-derived breast cancer models. This is an important advancement towards understanding hormone action in diverse tumors and a step towards the development of truly individualized medicine.